System and process for the recovery of propylene and ethylene from refinery offgases

ABSTRACT

A system and process for the recovery of ethylene and propylene from a gas stream, such as a refinery offgas stream. The process comprises withdrawing, cooling, and recovering an overhead vapor stream from a single distillation column. The process includes withdrawing a liquid C 3 +, C 4 +, C 5 + or C 6 + bottoms product stream and recycling a portion of the bottoms steam to maintain an overhead column condenser at temperature refrigeration levels of above −140° F. and withdrawing a vapor side stream rich in propylene or ethylene-propylene.

REFERENCE TO PRIOR APPLICATION

This application incorporates by reference and claims the benefit of thepriority date of U.S. Provisional Patent Application Ser. No.60/109,360, filed Nov. 20, 1998.

BACKGROUND OF THE INVENTION

Petrochemical offgases, such as refinery offgases from Fluid CatalyticCracker Units (FCCU) or Coker units, generally comprise an offgasmixture which comprises: hydrogen; nitrogen; carbon monoxide; ethane;ethylene; argon; propylene; as well as, butane and pentanes. Inparticular, the refinery offgasses from a FCCU contain olefincomponents, up to about 20 percent by volume ethylene and up to about 11percent by volume propylene, which components normally are not recoveredfrom the offgases, but which components may have value to warrantrecovery and use in other petrochemical processes or uses in downstreamprocessing. The typical range of components is listed in Table 1.

Normally, such refinery offgases are burnt and used as fuel. When therefinery FCCU is located near an ethylene or propylene plant, thenrecovery and downstream use of the recovered ethylene and propylene orpropylene is economical. In such recovery processes, ethylene andpropylene are recovered together and then directly or separatelyrecovered from the mixture.

In the publication, Chemical Engineering, May 1999, pages 30-33,“Refiners Get Cracking on Petrochemicals” hereby incorporated byreference, the attributes of Fluid Catalytic Cracker (FCC) technologyare explained. Propylene is increasing in demand, since it is afeedstock for polypropylene. About 30 percent (30%) of the global demandis a byproduct of FCC operations for producing gasoline. Over the nextfive years, the growth in demand is anticipated at over 5 percent (5%)per year. In response to this demand, FCC process technology is beingmodified to enhance olefin's production. Thus, the current yield of 5percent (5%) of the feedstock converted to propylene is being increasedto about 35 percent (35%), while the ethylene yield is increased fromabout 1 to 4 percent (1-4%).

Another option is the reaction of ethylene with 2-butene to producepropylene, termed metathesis. Thus, the recovery of these componentsfrom a FCC provides the opportunity for further propylene yields.

U.S. Pat. No. 5,546,764, issued Aug. 20, 1996, and incorporated hereinby reference, discloses an absorption process for recovering ethylenefrom a feed gas stream in an absorber stripper employing a heavyhydrocarbon absorption solvent, the absorber stripper bottoms stream isthen fractionated to produce an overhead, ethylene product stream.

The recovery of valued olefin components from offgasses may beaccomplished by partial condensation from the offgas feed stream;however, such recovery requires low temperatures, e.g., about —150° F.or lower. These low temperatures require refrigeration supplied byturboexpander processes and heat exchangers with multi-pass plate finheat exchangers. This type of equipment is not used extensively in oilrefineries. Furthermore, the use of such low temperatures increases thepossibility of gum formation by the reaction of NOX compounds withbutadiene, which gums may be explosive when the plant is shut down andwarmed up for restart.

U.S. Pat. No. 5,345,772, issued Sept. 13, 1994, shows a single columnprocess with Natural Gas Liquid (NGL) recycled to the upper zone of thecolumn. This process is applied to the recovery of the paraffiniccomponents of propane or ethane from gas streams with a high CO₂content. These gases are encountered in enhanced oil recovery projectswhere the CO₂ content of the gas stream may be 83 percent (83%), asshown in the patent example.

It is desirable to provide a system and process for the recovery ofolefins from offgases at warmer temperatures, such as propanerefrigeration levels, to avoid the disadvantages of prior art recoveryprocesses and systems and to provide other economic and processefficiency advantages.

SUMMARY OF THE INVENTION

The invention relates to a system and process for the recovery ofolefins from petrochemical offgases. In particular, the inventionconcerns the recovery of ethylene and propylene or propylene fromrefinery offgases from a Fluid Catalytic Cracker Unit (FCCU) of arefinery at propane refrigeration temperatures.

The process provides for maintaining the overhead condenser temperatureby the use of the liquid bottoms recycle stream from the single column.Generally, the overhead condenser is maintained at above −140° F., butdepending on the olefin component to be recovered, the overheadcondenser temperature may range, for example, from above about −114° F.for ethylene recovery or above about −35 to −40° F. for propylenerecovery, or within a range of about −20° F. to −114° F. for a mixtureof ethylene and propylene.

The system and process generally comprises a single distillation columnto receive an olefin-containing, hydrocarbon gas stream and maintainingthe overhead condensation temperature at a warmer temperature than −150°F., such as, at a refrigeration temperature level of above −140° F., byrecycling a heavy C₃+ bottoms stream from the single column, to theoverhead condenser or upper section of the distillation column, andwithdrawing a liquid bottoms stream; a vapor side stream rich in theselected olefin; and an overhead vapor stream lean in the olefincomponents.

The system and process of the invention comprises a system and processfor the recovery of olefin components, such as: ethylene; propylene; orcombinations thereof from a refinery offgas, such as: a FCCU; areformer; a Coker offgas; or other olefin gas source, which processcomprises: introducing the gas feed stream into a recovery distillationcolumn with an overhead condenser; withdrawing a liquid bottoms productstream from the recovery column; recycling at least a portion of theliquid or lower section bottoms product stream into the overheadcondenser or into an upper tray section, e.g., top 5 to 10 trays of therecovery column, to warm the overhead condenser; maintaining with therecycled product stream, the overhead condenser at selected propane,propylene, or ethylene refrigeration levels of above −140° F. andgenerally above −114° F.; withdrawing a heavy liquid product stream fromthe recovery column; withdrawing a vapor phase side downstream from thecolumn, which side downstream is rich in the olefin components to berecovered; and condensing and recovering the selected olefin componentsfrom the vapor phase side downstream, for downstream processing use; andwithdrawing an olefin lean, overhead vapor stream from the overheadcondenser or an upper section of the recovery column; and optionally,employing the lean overhead vapor stream as regenerated gas in thedehydration of the feed gas stream, fuel use, or other use.

Typically, and optionally, the refinery offgas feed stream processincludes: pretreating steps of water washing to remove impurities;removing acid gases, like carbon dioxide and hydrogen sulfide; anddehydrating the offgas feed stream. The process may be directed to therecovery of propylene or ethylene-propylene, generally, for downstreamprocessing use, such as, where an ethylene-propylene petrochemical plantis located near the offgas source.

The system and process provide multiple economic and process advantagesover prior art systems and processes to include, but not be limited to:eliminating the requirement of a separate column to produce the C₄+recycle stream, while only propane level refrigeration is required insome instances for the system and process. The system and processeliminate the formation of dangerous and explosive prior art gumformation, because of the relatively warm refrigeration leveltemperatures employed.

When propane refrigeration alone is used, the process permits the use ofa carbon steel column and carbon steel shell and tube heat exchangers,due to the propane or higher temperature refrigeration levels and noexpensive turboexpanders are employed. While in one embodimentillustrated, acid gas removal occurs in the offgas feed stream, the acidgas need not be removed upstream of the recovery column, because ofpotential freezing in the recovery column as in prior art cryogenicsystems, but may be removed downstream of the recovery column. In thesystem and process, less severe dehydration of the offgas feed stream isrequired than for a low temperature cryogenic system, so thatglycol-based dehydration may be used, e.g., the DRIZO® process (DRIZO isa registered trademark of OPC Design, Inc. for a gas dehydrationsystem). The system and process provide for high olefin recovery, forexample, from a FCCU offgas, about 98%+ for propylene, with low C₂/C₃ratios of less than about 0.01, and about 90%+ for ethylene.

The process and system can be used for the recovery of ethylene andethane for the offgases; however, the utility consumption will behigher, and the C₂ components of the offgas now become the sidedownstream along with the C₃ components.

The system and process shall be described for the purposes ofillustration only in connection with certain embodiments; however, it isrecognized that various changes, additions, improvements andmodifications to the illustrated embodiments may be made by thosepersons skilled in the art, all falling within the spirit and scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic illustration of the process and system forthe recovery of propylene from a Coke or FCCU offgas source.

DESCRIPTION OF THE EMBODIMENTS

The same general system and process configuration in the drawing can beemployed for either propylene (and heavier components) recovery orethylene (and heavier components), including propylene recovery.

In the drawing, process steps are designated by the 10, 11, 12, 13series. Process equipment within step 13 is designated by the 20, 21 . .. series. Process streams within step 13, designated by the 1, 2 . . .series.

The refinery offgases considered in this described embodiment are thecombined FCCU and Coker gases. The gases are at low pressure, nearatmospheric pressure, they are compressed to about 270 psig incompressor 10, cooled in exchanger 11 to 100° F., and then processed instages in a pretreatment step 12. These stages may be comprised of awaterwash; an amine contactor column for H₂S removal or other acid gasremoval; and a dehydration stage for water vapor removal. The treatedgas stream now enters the single column process 13.

The following is a description of the single column process forpropylene recovery. The feed vapor 1 enters column 20 at tray 16. Traydesignations are theoretical as encountered in the computer simulations.Tray 1 is the top tray, in this case, the reflux condenser 21. The twophase liquid-vapor stream from the overhead reflux condenser 21 isdirected to a reflux drum 22 for reflux and separation. The separated,reflux liquid stream from reflux drum 22 is returned to the upper traysection of the column 20. The separated, overhead vapor stream 6 iswithdrawn as an olefin-lean product stream.

The column has a side reboiler 23, for heat conservation purposes, attray 33. A side vapor draw, stream 8, is extracted from the column 20 atthis stage. The use and positioning of this side reboiler is importantin concentrating the side draw in the desired propylene to be recoveredand minimizing the ethylene component. The side reboiler 23 employsliquid from an intermediate tray of the column, below the point ofintroduction of the feed stream, and then after reboiling, returning thereboiled liquid to the tray below the tray from which the liquid iswithdrawn. The use of a column intermediate side reboiler 23 enhancesthe concentration of the olefin component in the vapor side draw stream8. The vapor side draw stream 8 is withdrawn from between the twointermediate trays used for the side reboiler 23.

The bottom reboiler 25 is tray 43. The bottoms liquid, stream 9, ispumped by 27, then cooled in 26 and split into two streams 4 and 5.Stream 5 is cooled and recycled to the condenser 21, joining the columnoverhead vapor stream at the condenser inlet. This stream is partiallycondensed by refrigerant in the reflux condenser 21. The vapor stream 6,and liquid stream 7, are separated in the reflux drum 22. The reflux isreturned to the column 20. The vapor is reheated against the recycledstream 5 in the exchanger 27, simultaneously chilling this stream to−20° F.

The vapor phase side draw, stream 8, is cooled and condensed inexchanger 24; stream 3 is then the light hydrocarbon product. The splitoff stream 4 is exported as the heavy liquid hydrocarbon product. Thereheated stream 2 goes to the refinery fuel gas stream.

The operating conditions for the column are listed in Table 2, and theoverall material balance and the recycle stream flow and composition isgiven in Table 3.

The following features of this column simulation are important for thefollowing reasons:

1) The column is operated at relatively low pressure, 250 psig.

2) The column's reflux condenser operates at -35° F. This level ofrefrigeration can be supplied by propane or propylene. The materials ofconstruction can be carbon steel for the entire system.

3) All the propylene recovered is in the light liquid hydrocarbonstream, and 98 percent (98%) of the propylene in the feedstock isrecovered in this stream. The C₂ and lighter components are rejected tothe fuel gas. The C₂ specification for stream 8 is:$\frac{{C_{2}H_{6}} + {C_{2}H_{4}}}{{C_{3}H_{8}} + {C_{3}H_{6}}} = 0.005$

4) The heavy liquid stream contains C₄+ components, and it ispredominately C₆+ and has a molecular weight of 79.4. These componentsare available in the feed gas. The composition of this recycled streamand net heavy liquid product is dependent on their relative quantitiesin the feed. The recycle to feed ratio is 0.18 moles/mole.

5) The coldest temperature in the tower, i.e., −35° F., is well aboveany potential gum deposition temperature, i.e., −150°0 F.

The single column, ethylene recovery process flow scheme is the same asshown in the drawing. The column operating conditions are altered toachieve the high ethylene recovery. The column conditions are listed inTable 4. The overall material balance and recycle stream flow andcomposition are listed in Table 5.

The features of this operation are:

1) The column operates at 250 psig, which is a considerably lowerpressure than that disclosed in U.S. Pat. No. 5,546,764, issued Aug. 20,1996, in which the absorber is operated at 550 psig, which requireshigher power consumption for the feed gas compressors.

2) The overhead temperature is −114° F. A cascade refrigeration systemis required for this system, and stainless steel materials are requiredwhere the lower temperatures are encountered.

3) The ethylene recovery is about 90 percent (90%).${{The}\quad \frac{{CH}_{4}}{{C_{2}H_{6}} + {C_{2}H_{4}}}\quad {ratio}} = 0.0025$

in the recovered light liquid stream. This temperature can be set toeliminate the need for downstream demethanization of the purifiedethylene.

4) The recycle stream is comprised of C₃+ components, which areavailable in the feed. A net purge, stream 4, maintains the materialbalance for these components. The recycle to feed ratio is 0.47moles/mole.

5) The lowest temperature in the system is above the gum formation anddeposit temperature.

The cited specific examples (supra) for propylene and ethylene recoverysupply the details of stream compositions and flows and the columnconditions. It is recognized that the system and process may be employedto any olefin recovery design process requirement, as appropriate.

The range of conditions for which the column may be designed is asfollows:

Pressure, psig 150 to 550 Condenser temperature, ° F. −140 to 0 Recyclestream composition C₃+, C₄+, C₅+ or C₆+ Recycle/feed ratio, mole/mole0.03 to 1.0 Olefin components recovered C₃H₆+ or C₂H₄+

Another embodiment of the process and system of the invention is therecovery of ethylene from synthesis gas produced by the steam crackingof hydrocarbons in an ethylene production facility. A typical gascomposition contains:

Typical Ethylene Plant Syngas Gas Percentage/volume H₂ 23 C₁ 19 C₂H₄ 32C₂H₆ 13 C₃H₆  7 C₃H₈  1 C₄+  3

The ethylene plant syngas is similar to the component range shown forrefinery offgases and is within the capability of the process and systemof the invention.

TABLE 1 FCC and Coker Offgas Component Concentration Ranges: ComponentPercentage Volume H₂ 7 to 22 CO₂ 0 to 2 H₂S 2 to 11 N₂ 1 to 17 CH₄ 25 to45 C₂H₆ 12 to 16 C₂H₄ 2 to 20 C₃H₈ 0.5 to 8 C₃H₆ 2 to 11 i, nC₄H₁₀ 1 to2 Butenes 0.4 to 4 C₅ 1 to 3 CH₄

TABLE 2 Single Column Operation Propylene Recovery HEATER/ PRESSURETEMPERATURE DUTY TRAY COOLER PSIG ° F. MMBTU/HR  1 CONDENSER 250 −35 5.116 FEED 257  50 33 SIDE REBOIL 258 175 4.9 VAPOR DRAW 43 REBOILER 260345 12.3 

TABLE 3 Single Colmnn Material Balance Propylene Recovery Stream ID 1 23 4 5 Name Feed Fuel Gas Light Liquid Heavy Liquid Recycle Phase MixedDry vapor Dry Liquid Dry Liquid Dry Liquid Fluid Rates, lb/mol/hr  1 H₂O.0000 .0000 .0000 .0000 .0000  2 H₂S .0295 .0288 6.4850E-04 7.6394E-1O1.0955E-08  3 N₂ 101.0916 101.0917 4.0918E-16 .0000 .0000  4 CO 15.352115.3521 1.7130E-16 .0000 .0000  5 CO₂ 14.7452 14.7452 2.3822E-061.1595E-14 1.6628E-13  6 H₂ 287.0592 287.0594 .0000 .0000 .0000  7 C₁1108.8446 1108.8454 8.5987E-10 4.6351E-20 6.6471E-19  8 Ethylene302.8797 302.8713 8.6336E-03 5.9396E-10 8.5179E-09  9 C₂ 486.4302484.5020 1.9285 1.1477E-06 1.6459E-05 10 Propylene 254.2555 5.1181249.1017 .0349 .5000 11 C₃ 138.7629 .4241 138.2981 .0402 .5769 12Isobutene 18.1834 .3218 17.3523 .5092 7.3024 13 lButene 25.3941 .480524.1390 .7745 11.1070 14 T2Butene 17.9134 .4505 16.4713 .9915 14.2194 15C2Butene 12.9536 .3417 11.7580 .8538 12.2442 16 13Butd .3810 7.2268E-03.3614 .0124 .1782 17 IC₄ 31.2459 .4924 30.1260 .6275 8.9985 18 NC₄23.5549 .7016 21.6213 1.2319 17.6671 19 3M1Butene .3733 .0124 .2993.0616 .8838 20 1Pentene 6.1519 .1679 4.6490 1.3349 19.1431 21 2M1Butene3.1259 .0817 2.3595 .6846 9.8181 22 2M2Butene 5.7787 .1175 4.0864 1.574822.5843 23 T2Pentene 5.0993 .1166 3.7056 1.2771 18.3152 24 C2Pentene2.9111 .0658 2.1088 .7365 10.5618 25 IC₅ 17.3760 .5095 13.2697 3.596751.5801 26 NC₅ 8.2941 .2025 6.0681 2.0235 29.0189 27 1Hexene 22.9652.2497 13.0143 9.7010 139.1198 28 NC₆ 14.8743 .1235 7.9553 6.7954 97.452229 NC₇ 5.6047 .0152 2.0050 3.5845 51.4047 30 NC₈ 1.1669 8.4490E-04 .2554.9107 13.0599 31 NC₉ .1702 2.30B6E-05 .0216 .1487 2.1321 32 NC₁₀ .04631.4052E-06 3.2544E-03 .0431 .6178 Total Rate, 2933.0148 2324.4973570.9684 37.5492 538.4857 lb-mol/hr Temperature, ° F. 100.0 81.0 100.0100.0 100.0 Pressure, psig 260 246 254.7 252 300 Molecular weight26.2661 19.7302 49.3807 79.3944 79.3944

TABLE 4 Single Column Operation Ethylene Recovery HEATER/ PRESSURETEMPERATURE DUTY TRAY COOLER PSIG ° F. MMBTU/HR  1 CONDENSER 250 −114  2.8 17 FEED 257  17 27 SIDE REBOIL 258 155 13.9 VAPOR DRAW 44 REBOILER260 337 20.1

TABLE 5 Single Column Material Balance Ethylene Recovery Stream ID 1 2 34 5 Name Feed Fuel Gas Light Liquid Heavy Liquid Recycle Phase Mixed Dryvapor Dry Liquid Dry Liquid Dry Liquid Fluid Rates, lb/mol/hr  1 H₂O.0000 .0000 .0000 .0000 .0000  2 H₂S .0295 1.4165E-08 .0295 6.3460E-108.7321E-08  3 N₂ 101.0916 101.0918 8.3535E-06 .0000 .0000  4 CO 15.352115.3521 3.9611E-06 .0000 .0000  5 CO₂ 14.7453 8.9805 5.7650 5.3078E-127.3035E-10  6 H₂ 287.0592 287.0596 1.2819E-09 .0000 .0000  7 C₁1108.8446 1106.9524 1.8937 2.6010E-16 3.5790E-14  8 Ethylene 302.879731.7627 271.1254 2.8096E-08 3.8660E-06  9 C₂ 486.4302 .0118 486.43342.6369E-06 3.6284E-04 10 Propylene 254.2555 .0605 254.1671 .0358 4.927311 C₃ 138.7629 .0910 138.6083 .0681 9.3670 12 Isobutene 18.1834 .456717.8763 .2600 35.7821 13 lButene 25.3941 .0635 24.9574 .3725 51.2536 14T2Butene 17.9134 .0380 17.5477 .3269 44.9778 15 C2Butene 12.9536 .023812.6742 .2549 35.0752 16 13Butd .3810 9.1258E-04 .3743 5.8301E-03 .802317 IC₄ 31.2459 .1028 30.7498 .3930 54.0781 18 NC₄ 23.5549 .0588 23.0823.4128 56.8034 19 3M1Butene .3733 4.9747E-04 .3606 .0121 1.6673 201Pentene 6.1519 5.3771E-03 5.8889 .2567 35.3160 21 2M1Butene 3.12592.5028E-03 2.9901 .1328 18.2728 22 2M2Butene 5.7787 2.9723E-03 5.4626.3120 42.9258 23 T2Pentene 5.0993 03.2742E-03 4.8462 .2489 34.2422 24C2Pentene 2.9111 1.7979E-03 2.7650 .1438 19.7891 25 IC₅ 17.3760 .016916.6683 .6883 94.7113 26 NC₅ 8.2941 6.1164E-03 7.8942 .3923 53.9813 271Hexene 22.9652 5.2600E-03 20.7222 2.2284 306.6281 28 NC₆ 14.87432.5264E-03 13.2102 1.6546 227.6777 29 NC₇ 5.6047 1.9756E-04 4.37241.2264 168.7467 30 NC₈ 1.1669 7.1470E-06 .7216 .4426 60.8989 31 NC₉.1702 1.2484E-07 .0722 .0972 13.3781 32 NC₁₀ .0463 3.0275E-09 .0118.0342 4.7007 Total Rate, 2933.0149 1551.7443 1371.2705 10.0000 1376.0029lb-mol/hr Temperature, ° F. 100.0 85.5 100.0 100.0 100.0 Pressure, psig270.5 246.0 253.9 252 300 Molecular weight 26.2661 14.7674 38.887679.6262 79.6262

What is claimed is:
 1. A process for the recovery of an olefin from agas feed stream comprising hydrocarbons and minor amounts of olefins,which process comprises: a) introducing the gas feed stream into asingle recovery distillation column having an upper tray section andwith an overhead condenser and reflux drum; b) withdrawing an overheadvapor stream from said column; c) cooling and partially condensing theoverhead vapor stream in the overhead condenser, to provide an overheadvapor-liquid; d) phase-separating the overhead vapor-liquid in thereflux drum; e) withdrawing from the reflux drum, an olefin-lean,overhead vapor stream and recycling the separated liquid from the refluxdrum to the column; f) withdrawing from said column, a liquid C₃+, C₄+,C₅+, or C₆+ bottoms product stream; g) recycling at least a portion ofthe liquid bottoms product stream to the overhead condenser or the uppertray section of said column, to maintain the temperature of the overheadcondenser above about −140° F. refrigeration temperature; and h)withdrawing an olefin-rich, vapor phase, side product stream rich in aselected olefin.
 2. The process of claim 1 wherein the gas feed streamcomprises a refinery offgas stream derived from a Fuel Catalytic CrackerUnit (FCCU), or Coker unit, or mixture thereof.
 3. The process of claim1 which includes pretreating the gas feed stream by waterwashing,removing acid gases, and dehydrating the gas feed stream.
 4. The processof claim 1 wherein the selected olefin comprises ethylene, propylene, orethylene and propylene mixtures.
 5. The process of claim 1 wherein thegas feed stream comprises up to about 20 percent (20%) by volume ofethylene and up to about 11 percent (11%) by volume of propylene.
 6. Theprocess of claim 1 for the recovery of propylene, ethylene, orethylene-propylene mixtures, which process includes employing a carbonsteel column, carbon steel shell and tube heat exchangers, and theprocess is free of employing turboexpanders.
 7. The process of claim 1which includes recovering, in the olefin-rich product side stream,greater than about 98 percent (98%) by volume of propylene in the gasfeed stream and greater than about 90 percent (90%) by volume ofethylene in the gas feed stream.
 8. The process of claim 1 whichincludes employing the overhead vapor stream for regeneration gas indehydrating the gas feed stream or as a fuel gas.
 9. The process ofclaim 1 which includes condensing the olef in-rich vapor side stream andrecovering use ethylene, propylene, or ethylene-propylene mixtures. 10.The process of claim 1 wherein a selected olefin comprises ethylene,propylene, or ethylene-propylene mixtures, and heavy liquid bottomsstream comprises primarily C₃+ to C₄+ hydrocarbons.
 11. The process ofclaim 1 wherein the selected olefin comprises propylene, ethylene, orethylene-propylene mixtures, and the liquid recycle stream comprisesprimarily C₅+ to C₆+ hydrocarbons.
 12. The process of claim 1 whichincludes maintaining the overhead condenser at a temperature range ofabout 0° F. to 140° F.
 13. The process of claim 1 which includesrecycling the column bottoms liquid stream in the molar ratio range from0.03 to 1.0, with reference to the feed stream.
 14. The process of claim1 wherein the single column operates at a pressure of about 150 to 550psig.
 15. The process of claim 1 wherein the selected olefin comprisesethylene and which process includes maintaining the overhead condensertemperature at about −114° F. or warmer.
 16. The process of claim 1wherein the selected olefin comprises propylene and which processincludes maintaining the overhead condenser temperature from about −40°F. or warmer.
 17. The process of claim 1 wherein the feed gas steamcomprises a synthesis gas stream produced by steam cracking ofhydrocarbons for the production of ethylene.
 18. The process of claim 1which includes, intermediate the column in a side reboiler and below thepoint of feed stream introduction withdrawing, reboiling, and recyclinga liquid side stream, to enhance the concentration of the selectedolefin.
 19. The process of claim 1 which includes withdrawing, in theside product vapor stream, ethane and propane with the selected olefin.20. The process of claim 1 which includes removing the acid gas, in thegas feed stream, downstream of the recovery distillation column.
 21. Theprocess of claim 1 which includes employing the overhead vapor streamfor the dehydration of the gas feed stream.
 22. The process of claim 1wherein the operating pressure of the recovery distillation column isabout 250 to 260 psig.
 23. A process for the recovery of anethylene-propylene mixture or propylene from a refinery offgas feedstream, which process comprises: a) introducing a waterwashed,dehydrated, refinery offgas feed stream, with acid gases removed, into asingle recovery distillation column with an overhead condenser, refluxdrum, and distillation trays, the offgas feed stream comprising C₁+hydrocarbons, ethylene, and propylene; b) withdrawing an overhead vaporstream from the recovery column; c) cooling and partially condensing theoverhead vapor stream in the overhead reflux condenser, to provide avapor-liquid overhead stream; d) phase-separating the vapor-liquidoverhead stream in the reflux drum; e) withdrawing from the reflux drum,a lean, stripped, overhead vapor stream essentially free of ethane,propane, ethylene and propylene; f) withdrawing from, reboiling, andrecycling to the column below the feed stream introduction, a sidereboiler stream to enhance the concentration of the propylene andethylene; g) withdrawing a heavy liquid C₃+ to C₆+ product stream fromthe bottom of the recovery column and recycling at least a portion ofthe liquid product stream to the overhead condenser or upper traysection of the recovery distillation column, to maintain the temperatureof the overhead condenser at a level of about 0° F. to −115° F.; h)withdrawing a vapor side stream from the recovery column from traysbetween the side reboiler, the side stream comprising ethylene andpropylene or propylene; i) condensing the vapor side stream; and j)recovering the ethylene-propylene mixture or propylene as a condensedrecovery product stream.
 24. The process of claim 23 which includeswithdrawing the liquid side stream from an intermediate upper tray ofthe column, recycling the reboiled side stream to the next lower tray ofthe column, and withdrawing the olefin-rich side product stream frombetween said intermediate upper and lower trays.